1. Field of the Invention
This invention resides in the field of the desulfurization of petroleum and petroleum-based fuels.
2. Brief Summary of the Invention
Fossil fuels are the largest and most widely used source of power in the world, offering high efficiency, proven performance, and relatively low prices. There are many different types of fossil fuels, ranging from petroleum fractions to coal, tar sands, and shale oil, with uses ranging from consumer uses such as automotive engines and home heating to commercial uses such as boilers, furnaces, smelting units, and power plants.
Unfortunately, most fossil fuels contain sulfur, typically in the form of organic sulfur compounds. Sulfur causes corrosion in pipelines and in pumping and refining equipment, as well as the premature failure of combustion engines. Sulfur also poisons the catalysts used in the refining and combustion of fossil fuels. Due to its poisoning of the catalytic converters in automotive engines, sulfur is responsible in part for the emissions of oxides of nitrogen (NOx) from diesel-powered trucks and buses. Sulfur is also responsible for the particulate emissions (soot) from trucks and buses since high-sulfur fuels tend to degrade the soot traps that are used on these vehicles. One of the greatest problems caused by sulfur compounds is their conversion to sulfur dioxide when the fuel is burned. When released to the atmosphere, sulfur dioxide results in acid rain which is harmful to agriculture, wildlife, and human health.
The Clean Air Act of 1964 and its subsequent amendments address the problem of sulfur in fossil fuels by imposing sulfur emission standards. Unfortunately, these standards are difficult and expensive to meet. Pursuant to the Act, the United States Environmental Protection Agency has set an upper limit on the sulfur content of diesel fuel of 15 parts per million by weight (ppmw), effective in mid-2006, a severe reduction from the standard of 500 ppmw as of the filing date of the present application. For reformulated gasoline, the EPA has lowered the standard to 30 ppmw, effective Jan. 1, 2004, as compared to 300 ppmw as of the filing date of this application. Similar changes have been enacted in the European Union, which will enforce a limit of 50 ppmw on the sulfur limit for both gasoline and diesel fuel in the year 2005.
Because of these regulatory actions, there is a continuing need for more effective desulfurization methods. The treatment of fuels to achieve sulfur emissions low enough to meet these requirements is difficult and expensive, and this inevitably results in increased fuel prices which have a major influence on the world economy.
The principal method of fossil fuel desulfurization in the prior art is hydrodesulfurization, a process in which the fossil fuel is reacted with hydrogen gas at elevated temperature and pressure in the presence of a catalyst. This causes the reduction of organic sulfur to gaseous H2S, which is then oxidized to elemental sulfur by the Claus process. Unfortunately, a considerable amount of unreacted H2S remains, and this poses a serious threat to human health. Another difficulty with hydrodesulfurization is that when it is performed under the more stringent conditions needed to achieve the lower sulfur levels, there is an increased risk of hydrogen leakage through the walls of the reactor.
Hydrodesulfurization also has limitations in terms of the types of organic sulfur compounds that it can remove. Mercaptans, thioethers, and disulfides, for example, are relatively easy to remove by the process, while other sulfur-bearing organic compounds such as aromatic compounds, cyclic compounds, and condensed multicyclic compounds are more difficult. Thiophene, benzothiophene, dibenzothiophene, other condensed-ring thiophenes, and substituted versions of these compounds are particularly difficult to remove by hydrodesulfurization. These compounds account for as much as 40% of the total sulfur content of crude oils from the Middle East and 70% of the sulfur content of West Texas crude oil. The reaction conditions needed to remove these compounds are so harsh that attempts to remove them often cause degradation of the fuel itself, thereby lowering the quality of the fuel.
Of possible relevance to this invention are co-pending U.S. patent application Ser. No. 09/676,260, entitled xe2x80x9cOxidative Desulfurization of Fossil Fuels With Ultrasound,xe2x80x9d Teh Fu Yen et al., inventors, filed Sep. 28, 2000, co-pending U.S. patent application Ser. No. 09/812,390, entitled xe2x80x9cContinuous Process for Oxidative Desulfurization of Fossil Fuels With Ultrasound and Products Thereof,xe2x80x9d Rudolf W. Gunnerman, inventor, filed Mar. 19, 2001, and co-pending U.S. patent application Ser. No. 09/863,127, entitled, xe2x80x9cTreatment of Crude Oil Fractions, Fossil Fuels, and Products Thereof With Ultrasound,xe2x80x9d Rudolf W. Gunnerman et al., inventor, filed May 22, 2001. All three of these co-pending United States patent applications are incorporated herein by reference in their entirety for all legal purposes capable of being served thereby.
It has now been discovered that a fossil (i.e., petroleum-derived) fuel can be desulfurized by a continuous process that applies ultrasound to a multiphase reaction medium that contains the fuel, an aqueous fluid, and a dialkyl ether, the reaction medium spontaneously separating into aqueous and organic phases after the ultrasound treatment, thereby enabling the immediate recovery of the desulfurized fossil fuel as the organic phase by simple phase separation. The invention resides in a continuous flow-through system in which the fossil fuel, the aqueous fluid, and the dialkyl ether are fed as a multiphase aqueous-organic reaction medium to an ultrasound chamber in which ultrasound is applied to the mixture, and the reaction medium emerging from the chamber is allowed to settle into separate aqueous and organic phases. The organic phase then constitutes the desulfurized fuel which is readily removable from the aqueous phase by simple decantation. Unlike similar desulfurization processes of the prior art, this process achieves desulfurization without the addition of a hydroperoxide to either the fuel or the aqueous fluid.
The terms xe2x80x9cdesulfurizedxe2x80x9d and xe2x80x9csulfur-depletedxe2x80x9d are used herein interchangeably, and both are intended to encompass fuels that contain no sulfur in any form, i.e., neither molecular sulfur nor organic or inorganic sulfur compounds, or so little sulfur that its level would be undetectable by conventional methods of detection. The terms xe2x80x9cdesulfurizedxe2x80x9d and xe2x80x9csulfur-depletedxe2x80x9d are also used to include fuels whose sulfur content (either as molecular sulfur or as organic or inorganic sulfur compounds) is substantially reduced from that of the starting fossil fuel, and preferably to a level below any of the upper limits imposed or to be imposed by regulation as mentioned above.
Certain organic sulfur compounds that are typically present in fossil fuels are illustrative of the effectiveness of the process. These compounds are dibenzothiophene and related sulfur-bearing organic sulfides. These compounds are the most refractory organic sulfur compounds in fossil fuels. Although other explanations are possible, it is believed that these sulfides are converted to the corresponding sulfones by this process, the sulfones having greater solubility in the aqueous phase and therefore more readily removable by separation of the phases. The ultrasound-promoted reaction that occurs in the practice of this invention is selective toward the sulfur-bearing compounds of the fossil fuel, with little or no effect in the non-sulfur-bearing components of the fuel. The continuous flow-through nature of this invention permits a large quantity of fossil fuel to be treated at a modest operating cost and a low residence time in the ultrasound chamber. These and other advantages, features, applications and embodiments of the invention will be better understood from the description that follows.